Electric double layered capacitors (EDLC), commonly known as ultracapacitors, are known in the prior art. Such devices are capable of storing larger amounts of electrical energy per unit volume than the traditional capacitors, generating much higher power in a short instant than many types of chemical batteries, and may be charged and discharged a large number of times with virtually no energy losses due to chemical reaction. Ultracapacitors having capacitances of thousands of Farads are already commercially available, and are being used as energy storage devices for providing back-up currents for microcomputers, clock radios, and other consumer electronics, as well as actuators or primary power sources capable of providing sufficient current for automobile engine cranking, as well as power sources for hybrid and electrical vehicles.
Such devices achieve their relatively high capacitances by virtue of a high-area electrode microstructure. In conventional capacitors, the electrodes are typically metallic plates separated by a dielectric material. As capacitance is dependent upon the area of the electrode, such plate-type electrode structures must be made very large to obtain capacitances in excess of one F. Ultracapacitors circumvent this limitation by means of electrodes formed from very fine, particulate materials, such as activated carbon, having surface area to mass ratios on the order of 1,000 to 3,000 m2/gm. The resulting high surface area per unit volume that such electrode structures provide allow much higher capacitances to be stored in the resulting ultracapacitor than could possibly be stored in a capacitor using conventional, plate type electrodes.
In the simplest design of an ultracapacitor, two high-area electrodes are separated a short distance from one another via a dielectric material. Current collectors (which may be in the form of either wires or plates) are centrally provided within each of the particulate carbon electrode structures. The electrodes and the dielectric separating them are soaked in an electrolyte, which is preferably non-aqueous in order to avoid limitations on the charging voltage that are inherent with aqueous-type electrolytes. A charging voltage is then applied across the current collectors of the two opposing electrodes, which in turn allows a relatively large amount of positive and negative charges to migrate from the current collectors to the surfaces of the mutually contacting carbon particles forming the electrode structures. The charging process is complete when the capacitor is saturated. Electrical power may then be tapped from the current collectors as needed.
Unfortunately, such a simplistic, two-electrode carbon-based design does not yield efficient electrical power. While the power output may be increased by shortening the distance between the current collectors and the particulate carbon, the resulting lower volume of electrodes would of course reduce the available electrode area and hence the capacitance.
To overcome these limitations, ultracapacitors having multiple electrodes connected in parallel have been constructed. In one such design, an extruded honeycomb substrate formed from a conductive carbon-based material forms the first set of electrodes of the ultracapacitor, while monolithic carbon rods disposed in the hollow channels of the honeycomb structure form the other set of electrodes. To prevent short circuiting between the two sets of electrodes, the interior walls of the channels of the honeycomb structure are coated with a dielectric polymer film prior to the insertion or manufacture of the rod-like electrode structures within the honeycomb channels.
Because honeycomb substrate having relatively high channel densities of between 400 and 2,000 channels per square inch may be manufactured with existing extrusion technology, an ultracapacitor having hundreds or even thousands of electrodes are possible with this approach. The relatively small cross section where the cross sectional area of the resulting electrodes provides short distances between the centrally disposed current collectors, and the surrounding matrix of particulate carbon, allowing electrical charges to migrate to the surfaces of the particulate carbon with relatively small internal resistance, thereby resulting in an ultracapacitor that is chargeable within a matter of a few seconds, and which has a highly usable discharge energy.
Unfortunately, such honeycomb-type ultracapacitors have not yet realized their full potential in providing a low-cost, high energy storage device. It has been proven very difficult to install the rod-like, carbon electrode structures within the channels of the ceramic substrate. No practical and time-efficient method has yet been found to produce such rod-like electrode structures and to insert the hundreds or thousands required into the small, individual openings of the honeycomb channels. While extrusion techniques have been attempted, the small cross-sections of the channels and their close distances together has made it difficult to reliably and rapidly form electrode structures with current collectors within the channels without the formation of void spaces which compromises the performance and capacity of the resulting ultracapacitor. It has also proven difficult and time consuming to uniformly and reliably apply a coat of dielectric, insulating polymer over the interior walls of the hundreds or thousands of small channels within such honeycomb structures. Finally, the carbon based honeycomb substrates tend to be brittle and fragile, and thus prone to cracking or breakage during the installation of the rod-like electrode structures. The resulting cracks or other discontinuities create electrical leakages in the final ultracapacitor, which in turn degrade its performance.
Clearly, what is needed is an ultracapacitor capable of exploiting all the advantages of extruded honeycomb substrates without the accompaniment of any of the aforementioned disadvantages. In particular, a technique for manufacturing a honeycomb based ultracapacitor is needed wherein electrode structures are quickly and easily formed within each of the channels of the substrate without potentially wall-breaking forces and without the formation of performance compromising voids that reduce both energy and power performance. Ideally, the design of the honeycomb based ultracapacitor would obviate the need for coating the interior walls of the channels with a dielectric polymer. Finally, it would be desirable if the honeycomb substrate could be formed from a material stronger and more robust than carbon-based conductive compounds, and hence less apt to form internal cracks or other discontinuities that would compromise the performance of the final device.